In rotating machinery, various rotating elements such as compressor wheels, turbine wheels, fans, generators, and motors are affixed to a shaft upon which they rotate. The shaft can be a single piece unitary structure of nearly constant diameter or it can be a compound structure having two or more relatively rigid or stiff shaft elements connected by one or more relatively flexible shaft elements. A single piece shaft machine would typically have its shaft supported by two journal bearings and a bi-directional thrust bearing. A two stiff shaft element compound shaft machine would typically have each of its stiff shaft elements supported by two journal bearings (for a total of four journal bearings) and would have either one or two bi-directional thrust bearings (two thrust bearings being required if the relatively flexible shaft element coupling allowed sufficient axial flexibility and both sections require accurate axial position).
Until recently, the rotating machinery industry generally had considered that it was impractical to support high speed turbomachinery shafts of either the rigid or compound type on three journal bearings owing to the difficulty of holding three bearings in straight alignment, together with the large shaft and bearing stresses that result when bearing misalignment occurs. Recent improvements in flexible shaft elements have, however, made such combinations possible and single flexible disk diaphragm shafts have been successfully employed between two relatively rigid shafts supported by three bearings in straight alignment. An example of this type of structure can be found in United States patent application No. 08/440,541 filed May 12, 1995 by Robert W. Bosley entitled "Compound Shaft with Flexible Disk Coupling" now U.S. Pat. No. 5,697,848 issued Dec. 16, 1997.